This proposal is to request support for a Keystone Symposia meeting entitled "Dendritic Cells", organized by Gwendalyn J. Randolph and Sebastian Amigorena, which will be held in Banff, Alberta Canada from March 29 - April 3, 2009. The current studies on dendritic cells (DCs) have confirmed not only their major role as antigen presenting cells in adaptive immunity but also their important functions in the initiation of the innate resistance and inflammatory responses. DCs are thus an important bridge between innate resistance and adaptive immunity either through cellular interactions or secretion of pro-inflammatory and immunoregulatory cytokines. The origin and migration pattern of DCs as well as their sharing of hematopoietic lineages, functions, and receptors with other phagocytic cell types such as monocytes and macrophages is a subject of intense investigation. The special characteristics of antigen processing and expression of immunoregulatory gene products in DCs are central in their ability to either activate and tolerize T cells and on their in vivo functions in degenerative, autoimmune and infectious diseases as well as in cancer. In this symposium, the following aspects of DC biology will be emphasized: (1) DC precursors and relation with monocyte and macrophages;(2) receptors, phagocytosis, and antigen presentation;(3) regulation of type I IFN production;(4) DCs in tolerance, immunity, and T cell activation;and (5) DCs in cancer and other diseases. Opportunities for interdisciplinary interactions will be further enhanced by the concurrent meeting "Pattern Recognition Molecules and Immune Sensors of Pathogens", which will share a plenary session, a workshop, and a keynote with this meeting.